1. Field of the Invention
The present invention relates to a photoelectric conversion device intended to attain a good signal-to-noise ratio and to simplify the structure, the photoelectric conversion device being applied to the input unit for characters, figures, images and the like in facsimiles and digital copiers.
2. Description of the Prior Art
In a so-called line sensor having a plurality of light reception elements disposed in an array, particularly in an elongated line sensor, it is important to transfer input signals from respective light reception elements to succeeding signal processing units, image forming units, and image transference units, while retaining a high signal-to-noise ratio as well as reliably separating respective input signals.
FIGS. 1 and 2 are circuit diagrams showing a linear photoelectric conversion apparatus according to the prior art.
In the figures, light reception elements C.sub.i1 C.sub.i2, . . . C.sub.in (1.ltoreq.i.ltoreq.m) n in number constitute a single block, and blocks m in number constitute a photosensor array 1.
For the purpose of description herein, the subscript i of the light reception element C.sub.ij is used for indicating the block number, while the subscript (1.ltoreq.j.ltoreq.n) is used for indicating the number of a particular light reception element in the block including that element.
Referring now to FIG. 1, one terminal of the light reception element C.sub.ij is connected to a common electrode B.sub.i, one of which is provided for each corresponding block, while the other terminal, in combination with those having the same second subscript is connected to a corresponding separate electrode S.sub.1, S.sub.2, . . . , S.sub.n.
The common electrode B.sub.i is connected to a scanning circuit 2, and the separate electrode S.sub.j is connected to the input terminal of an amplifier A.sub.j of an amplifier unit 3. The output terminals of the amplifier A.sub.j are arranged such that any one of them is connected by means of a switch unit 4 to an analog/digital converter 5 from which a digital signal is derived.
The scanning circuit 2 sequentially selects the common electrode B.sub.i in response to shift pulses SH1 and applies a predetermined voltage V.sub.i to the block connected to the selected common electrode B.sub.i. Upon application of the voltage V.sub.i, the light reception elements C.sub.i1 to C.sub.in of the block are enabled.
Photocurrent corresponding in amount to the incident light intensity passes through the enabled light reception elements C.sub.i1 to C.sub.in, the photocurrent signals being amplified by the amplifiers A.sub.1 to A.sub.n. The amplified photocurrent signals are sequentially input by means of the switch unit 4 to the A/D converter 5 to thereby output a digital time-sequential signal.
Alternatively, in the circuit shown in FIG. 2, the output terminal of an amplifier A.sub.j is connected to an A/D converter AD.sub.j of which the output terminal is connected to the input terminal of SR.sub.j constituting a stage of a shift register 6.
In operation, upon selection of the common electrode B.sub.i by means of the scanning circuit 2, the light reception elements C.sub.i1 to C.sub.in are enabled so that each photocurrent signal is amplified by one of the amplifiers A.sub.1 through A.sub.n and in turn converted by the A/D converter AD.sub.1 through AD.sub.n into a digital form to be stored in SR.sub.1 through SR.sub.n of the shift register 6. In response to the shift pulses SH2 from the control section, the shift register 6 outputs the stored contents in the form of a time sequential signal. In the above circuit arrangement the time period from the time instant when the voltage V.sub.i is applied to the common electrode B.sub.i to the time instant when the photocurrent signals of the light reception elements belonging to the selected block are read out. is constant for each light reception element C.sub.i1 through C.sub.in.
In the prior art circuits shown in FIGS. 1 and 2, the scanning circuit 2 sequentially selects the common electrode B.sub.i and applies the predetermined voltage V.sub.i. In this case, the other, non-selected common electrodes (i.e., those other than B.sub.i) are supplied with zero potentials. Since the input potential to the amplifier A.sub.j is intended to be zero, the light reception elements connected to the non-selected common electodes are not applied with any voltages, so that no current is expected to flow. Thus, it is expected that the amplifier A.sub.j is input with only the photocurrent from the light reception element connected to the common electrode and thus the amplifier A.sub.j amplifies only the input photocurrent.
However, in practice, the following potential differences exist.
(1) It is difficult to drive the non-selected common electrodes so as to maintain their potentials completely at zero potential. Common bipolar CMOS IC driving causes 10 to 50 mV potentials.
(2) An input offset voltage is generated at the amplifier A.sub.j connected to the separate electrode. The voltage is generally on the order of .+-.10 mV which is added to the separate electrode.
Due to these additional voltages, a current flows through the non-selected light reception element and is superposed upon the photocurrent signal of the selected light reception element. Therefore, the resultant signal has crosstalk and a degraded S/N ratio.
Generally, in order to solve the above problems, a method has been used in which a blocking diode for blocking a crosstalk current is serially connected to each light reception element constituting the photosensor array 1. The blocking diode is commonly implemented as a Schottky diode on the same substrate as the light reception element, aiming at a reduction in the number of processes required for the implementation and at the realization of a compact dimension.
However, it is difficult to manufacture with a good yield such a diode with a small reverse leakage current and characteristics that are uniform over the length of the elongated sensor. This results in an extraordinarily low yield in manufacturing an elongated linear photoelectric conversion apparatus.